Process for the production of acrylic acid

ABSTRACT

The present invention relates to a process for the production of acrylic acid (AA) comprising the steps wherein: a) a 1st gas mixture comprising propylene, oxygen, an inert gas and steam is subjected to a 1st catalytic oxidation reaction stage thereby converting the propylene in the presence of a catalyst mainly into acrolein being contained in a 2nd gas mixture from said 1st catalytic oxidation reaction, b) said 2nd gas mixture from the 1st catalytic oxidation reaction stage is subjected to a 2nd catalytic oxidation reaction stage thereby converting the acrolein in the presence of a catalyst mainly into AA, being contained in a product gas, c) said product gas is subjected to a quench tower, wherein said AA is recovered as an aqueous solution comprising said AA being contained in the process water, wherein a process vent gas is obtained at the top of said quench tower, wherein, said 1st gas mixture has a steam/propylene ratio of &gt;0.3 and &lt;2 and, the amount of said process water is less or equal to the amount of water in said aqueous solution withdrawn from the quench tower.

This application is a national stage application under 35 U.S.C. 371 ofinternational application No. PCT/EP02/14227 filed Dec. 13, 2002, whichis based on European application No. EP 011 29 334.7 filed on Dec. 14,2001, and claims priority thereto.

The present invention relates to a process for the production of acrylicacid (AA) by catalytic vapor phase oxidation of propylene in two steps.The invention relates more particularly to a process for a veryefficient production of a highly concentrated aqueous AA solution byoxidation of propylene at high concentration with high production ratesand low emissions using recycled gas from a catalytic combustion unit bywhich nearly all organic compounds in the arising process vent gas andprocess water from propylene oxidation are removed.

The process for producing AA by the two-stage (the first catalyticoxidation reaction stage for conversion of propylene mainly to acroleinand the second catalytic oxidation reaction stage for conversion ofacrolein to AA) catalytic vapor phase oxidation of propylene usingmolecular oxygen is already known and used on an industrial scale forseveral decades. For several reasons (flammable limits propylene/air,high heat of reaction) it is necessary to dilute the reaction gases byinert gases (e.g. water vapor, N₂, CO₂).

A typical process for industrial production is as follows. A mixture ofpropylene, air and steam is supplied to a first oxidation reactor andthe propylene is converted mainly to acrolein and small amounts of AA inthe first step. The product is supplied to a second oxidation reactorwithout separation. Fresh air and steam, if required for the subsequentoxidation reaction in the second oxidation step, can be added at theinlet of the second oxidation reactor.

For the separation of the gaseous AA from water vapor in the app. 180°C. effluent of the oxidation reactors two separation process routes arein use:

-   -   1. absorption of the gaseous AA in a high boiling hydrophobic        aromatic solvent at temperatures that the process water will        remain in the process vent gas leaving on the top of an        absorption tower (see e.g. DE 43 08 087/BASF, 15 Sep. 1994)    -   2. absorption of the AA in water with simultaneous quenching to        low temperatures in a quench tower/collector in such a way that        nearly all of the vaporized process water in the 180° C.        effluent of the second reactor will be condensed (see e.g. DE 30        42 468/Mitsubishi Chem. Corp. (MCC), 11 Nov. 1980).

Whereas in the first process route the AA and the high boiling productsare separated in several distillation steps in the second process routeafter quenching, the water in the aqueous AA is separated in asubsequent azeotropic distillation to get crude AA, from which highpurity AA for the production of AA copolymers or different AA esters areproduced.

In the second process route, which will be regarded here, the handlingof the process water, which consists of the water produced by theoxidation and the water necessary for dilution to be outside flammablelimits has a remarkable influence on the economics of the process.

When the product gas containing AA obtained at the outlet of the secondoxidation reactor is introduced into the quench tower to obtain AA as anaqueous solution, the resulting process vent gas containing unreactedpropylene and other low boiling organic material is leaving at the topof the quench tower and has to be treated e.g. in an incinerator so thatno or nearly no organic compounds are emitted into air (one pass throughprocess).

It is also possible that a part of the process vent gas is recycled andadded to the propylene/air/water stream at the inlet of the firstreaction stage (cycle gas flow process) which is a quite commonprocedure especially in the case where the process is run with partialpropylene conversion.

Improvements for this process have been proposed to produce AA moreefficiently on a large scale by vapor-phase catalytic oxidation ofpropylene. Special improvements in this context are:

-   -   recycling the process vent gas to increase the concentration of        AA in the bottom liquid of the quench tower by partly        substituting the H₂O vapor in the first stage reactor inlet        stream by N₂ to operate outside flammable limits of        propylene/air (see e.g. DE 30 02 829/MITSUBISHI PETROCHEMICAL        Co. (MPCL), 26 Jan. 1980) and especially    -   recycling the process water by separating the process water in a        subsequent water separation unit (e.g. by azeotropic        distillation). The process water is vaporized by using the heat        of the hot effluent from the second oxidation reactor and is        recycled as water vapor together with the process vent gas to        the inlet of the first reaction stage. The not recycled part of        this recycle stream is subjected to a thermal or catalytic        combustion unit (see e.g. EP 0 695 736 B1/MITSUBISHI CHEMICAL        Co., 4 Aug. 1995 or EP 0 778 255 A1/NIPPON SHOKUBAI Co., 5 Dec.        1996 or EP 0 861 820 A2/NIPPON SHOKUBAI Co., 27 Feb. 1998).    -   using under nearly complete propylene conversion a catalytic        combustion unit to oxidize the organic compounds in the process        vent gas from top of the quench tower upstream of the branch of        the treated process vent gas (see e.g. EP 0 274 681        B1/MITSUBISHI PETROCHEMICAL Co., 10 Dec. 1987).

To use a catalytic combustion unit for the treatment of the organiccompounds in the recycled process vent gas has several advantages.Besides that there is no special equipment (like a condenser) necessaryto separate the acids in the recycle gas (mainly acrylic-, acetic- andpropionic acid), which can damage the oxidation catalyst in the firstcatalytic oxidation reaction stage and reduce the life time of thiscatalyst, the NO_(x) emissions of a catalytic combustion unit are morethan 100 times less (<1 ppm) compared to a thermal combustion unitbecause of the much more lower temperature used during combustion (appr.550° C.).

Up to today the process route with a catalytic combustion unit in therecycle gas is used only with recycled process vent gas and withoutrecycling the process water. A further disadvantage is that the route isused only with low space time yield (STY) catalysts in the range of0.16-0.17 kg AA/liter catalyst*h or low propylene space velocities(SV_(P)) in the range of 70 N1 propylene/1 reaction volume 2^(nd)reaction stage. Together with a low organic combustion rate Pt catalystin the catalytical combustion unit (see EP 0 274 681 B1/MITSUBISHIPETROCHEMICAL Co., 10 Dec. 1987), only low AA concentrations in thebottom liquid of the quench tower have been achieved.

Object of the present invention is to convert propylene to AA with highproduction rates together with long catalyst life time for the first andfor the second catalytic oxidation reaction stage and together with highAA yield resulting in a high AA concentration in the quench tower bottomliquid in order to produce AA economically. Also treating the organiccompounds in the process vent gas and process water with lowest possibleNO_(x) emissions is a necessary environmental issue today.

In the range of AA concentrations>80% by weight in the bottom liquid theabsorption process of gaseous AA in the process water becomesineffective and uneconomic because of the high number of trays needed(pinch effect). Accordingly it is a further object of the presentinvention to circumvent said pinch effect (see examples below).

From EP-A-0 274 681 a two step process for the production of AA isknown. A gas mixture of propylene, molecular oxygen and an inert gas issubjected to 2 catalytic oxidation reaction stages. The resultingproduct gas is then subjected to an AA recovery step, in which the AA isrecovered as an aqueous solution. The vent gas obtained from the AArecovery step is subsequently subjected to catalytic combustion andpartially recycled to the 1^(st) catalytic oxidation reaction stage.However, this process has the disadvantage that the steam concentrationin the gas mixture which is subjected to the 1^(st) catalytic oxidationmust be as low as possible in order to increase the life of theMo—Bi-based catalyst. Additionally the AA concentration in the aqueoussolution is rather low by weight so that the subsequent separation ofthe water from the AA is very costly and a lot of waste water has to betreated. The space time yield (STY) obtained is 0.164 kg AA/litercatalyst*h.

GB-A-1539671 teaches a process for the production of AA from propylenevia acrolein as an intermediate by catalytic vapor phase oxidation. Theprocess comprises passing a starting reactant gas mixture through twocatalytic oxidation reactors and introducing the resulting AA containinggas into an AA collector, thereby recovering the AA in the form of anaqueous solution. The exhausted gas from the collector is partiallyincorporated into the starting gas mixture. The process has thedisadvantage that the recycled gas contains traces of unrecovered AA andacetic acid which deactivate the Mo—Bi based catalyst.

EP-A-0 778 255 discloses a process for producing AA by subjectingpropylene and/or acrolein to catalytic gas phase oxidation. The AAcontaining gas thus obtained is contacted in a quench tower with anaqueous collecting agent comprising AA, acetic acid and a poorly watersoluble solvent to recover the AA from the gas as an aqueous solution.This aqueous solution is then subjected to an azeotropic distillation inthe presence of a poorly water soluble solvent to obtain high purity AA.This process has the disadvantage that only up to 90% of the aqueoussolution resulting from the azeotropic distillation are recycled to thequench tower so that a part of the aqueous solution has to be treated aswaste water.

WO 99/14182 relates to a method for the fractional condensation of a gasmixture which contains at least one other condensable consistent inaddition to AA or methacrylic acid and which also has a high proportionof one or several non-condensable compounds. According to said method,the gas mixture is passed through a column with separation-efficientbaffles and the condensable constituents are condensed out by cooling.However, this teaching suffers under a highly concentrated bottom liquidstream which contains AA of 40% by weight, is rejected and needs a highreflux ratio on column top and a high bottom temperature.

It is therefore the objective of the present invention to provide asuperior industrial process for the production of AA which overcomes thedisadvantages of the state of the art. This objective is achieved by theprovision of a process for the production of AA comprising the stepswherein:

-   -   (a) a 1^(st) gas mixture comprising propylene, oxygen, an inert        gas and steam is subjected to a 1^(st) catalytic oxidation        reaction stage thereby converting the propylene in the presence        of a catalyst mainly into acrolein being contained in a 2^(nd)        gas mixture from said 1^(st) catalytic oxidation reaction,    -   (b) said 2^(nd) gas mixture from the 1^(st) catalytic oxidation        reaction stage is subjected to a 2^(nd) catalytic oxidation        reaction stage thereby converting the acrolein in the presence        of a catalyst mainly into AA, being contained in a product gas,    -   (c) said product gas is subjected to a quench tower, wherein        said AA is recovered as an aqueous solution comprising said AA        being contained in the process water, wherein a process vent gas        is obtained at the top of said quench tower, and optionally    -   (d) said process water, separated in a subsequent separation        unit (e.g. azeotropic distillation) is fed back into said quench        tower so that most parts of the process water vaporized is mixed        with the process vent gas leaving the quench tower on top and is        treated together with the process vent gas in a subsequent        thermal or catalytic combustion unit,        characterized in that    -   said 1^(st) gas mixture has a steam/propylene molar ratio        of >0.3 and <2 and    -   the amount of said process water is less or equal to the amount        of water in said aqueous solution withdrawn from the quench        tower.

The foregoing and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawing where:

FIG. 1 is a schematic of the process to make acrylic acid of the presentinvention.

In a preferred embodiment of the present invention the process for theproduction of AA comprising the steps (a), (b), (c) and (d).

In a preferred embodiment of the present invention all process waterthat results from the separation of the AA contained in the aqueoussolution is treated in a combustion unit, so that the entire AAproduction process is free from residual waste water which otherwise hadto be treated in a subsequent unit (e. g. thermal or catalyticcombustion unit, activated sludge). The process water is preferablyadded to the top of the quench tower for absorbing the AA out of theprocess gas of the 2^(nd) oxidation reaction stage.

Furthermore, it is preferred according to the invention that in a step(d) said process vent gas and the vaporized process water is subjectedto a catalytic combustion unit yielding a combusted process vent gas andat least a part of said combusted process vent gas is recycled to said1^(st) catalytic oxidation reaction stage.

The recycle gas ratio (rgr) is defined as molar ratio of stream 3(recycle gas feed into inlet of first oxidation reaction stage) to thesum of propylene feed (stream 1), oxygen feed into inlet of firstoxidation reaction stage and water feed into inlet of first oxidationreaction stage. The combusted process vent gas is recycled with a rgr ofpreferably 0.1-0.2, more preferably 0.13-0.25 and most preferably0.15-0.40.

According to an embodiment of the present invention, the gas mixturewhich is subjected to the 1^(st) catalytic oxidation reaction stagecomprises oxygen, propylene, an inert gas and steam. The inert gas andsteam concentration at the 1^(st) stage reactor inlet is determined bythe recycle gas stream. The concentration of the propylene in the gasmixture at the 1^(st) catalytic oxidation reaction stage inlet ispreferably >9 Vol.-%, more preferably >11 Vol.-% and most preferably >14Vol.-%. The molar ratio of oxygen/propylene is in the range of 1-3,preferably 1.2-2.5 more preferably 1.4-1.8 and most preferably 1.3-1.7.The molar ratio of steam/propylene is, according to the presentinvention, >0.3 and <2, preferably 0.5-1.5. The steam can be taken froma steam supply and is added to the gas mixture. Furthermore, it ispreferred that said process water is at least a part, preferably atleast 90% by weight, of the water separated from the aqueous solutioncomprising water and AA.

According to another embodiment of the present invention, the steam inthe combusted gas is recycled to the 1^(st) catalytic oxidation reactionstage. The steam content in the combusted gas is adjusted by thetemperature at the top of the quench tower and the amount of organiccompounds in the process vent gas. The amount of water and the amount ofinert gas in the gas mixture have to be adjusted such that the gasmixture is outside the exposable limits.

As inert gases N₂, H₂O and CO₂ are preferred. More preferable, however,the combusted gas which comprises mainly N₂, CO₂ and steam is used asinert gas.

For the oxidation of the organics to CO₂ in the catalytic combustionunit it has been found, that catalysts based on titanium and oxygencomprising compounds as carriers, preferably catalysts based onTiO₂-carriers, are extraordinarily stable under these processconditions, superior to all other carriers with respect to long termthermal and chemical stability and resistance against thermal shock athigh dispersion and, thus, high specific long term activity of theprecious metal dispersion. Additionally it has been found, that catalystcarriers from anatase are especially advantageous because the catalystsprepared with these carriers show the highest long term stability,higher than with rutile or brookite based carriers.

Oxidation reactions on precious metals are very fast and thus undergostrong mass transport limitations if the precious metal is deposited allover the surface of the porous structure of the carrier. This occurseasily when the impregnation is simply done by soaking the carrier witha solution of a very stable precious metal compound. One furtherpreferred feature of the oxidation catalyst is that the precious metalis applied in a thin shell. Commonly, if the catalyst carrier is notsufficiently reactive to precipitate the precious metal, e.g. byneutralization of an acidic solution, the carriers are impregnated withsoluble alkaline compound in a primary step. Mostly, the precious metalcompound is in this case of chloridic nature, e.g. hexachloroplatinicacid.

For oxidation reactions, however, the presence of chloride residues isdetrimental because the oxidation is extremely inhibited. Therefore, thechlorides must carefully be washed out of the catalyst in this case.This way of “fast” precipitation of the precious metal is applicable forthe impregnation of particulate catalysts. If monoliths in honeycombform have to be impregnated, a lot of problems are obtained, e.g. byenrichment of the precious metal in the respective ends of the channels,leaving the middle of the channels nearly unimpregnated. This problem isless severe applying a dense, pore free monolith with a washcoat.

Because the reaction involves the handling of large volumes of off gasunder recycling, very low pressure losses within the reactors aredesired. This may, on the other side, preferably be achieved by theapplication of honeycomb shaped monolith carriers, which then arepreferably equipped with precious metal in a thin shell, but alsodisplay a very even distribution of the metal on the surface of thechannels. Preferably the honeycomb carrier has a low delta pressure ofless than 20 mbar/m³, preferably less than 10 mbar/m³ and mostpreferably less than 3 mbar/m³.

These catalysts can preferably be obtained by impregnation of thecarrier with a moderately unstable precious metal compound, establishingon one side an even distribution within the channels, on the other sidea precipitation of the precious metal before the soluble compounds arediffused into the core of the walls. This is preferably done byimpregnation with a metal nitrate solution of the metal used in thecatalyst, received by dissolution of hexachloroplatinic acid inconcentrated nitric acid. In a similar way also impregnation withmixtures of several precious metals may be done, e.g. by platinum andpalladium, either subsequently or simultaneously. The advantage of suchmixtures may be a lower inhibition of the catalyst by carbon monoxide, aphenomenon which is well known.

As catalysts for the 1^(st) stage catalytic oxidation reaction ofpropylene to acrolein usually Mo—Bi based catalysts are used. Mo—Co—Bibased catalysts are the more preferred catalysts. In the 2^(nd) stagecatalytic oxidation reaction from acrolein to AA Mo—V based catalystsare usually used and Mo—V—Bi catalysts are more preferred catalysts.Furthermore, it is preferred that the catalysts are of the compositeoxide type.

The catalytic oxidation reactions are preferably heterogeneouslycatalyzed reactions. The 1^(st) catalytic reactions takes placepreferably at a high temperature salt bath temperature >250° C., morepreferably in a range from 275 to 400° C. and most preferably in a rangefrom 290 to 330° C. and the 2^(nd) catalytic reaction is preferablycarried out at a high temperature salt bath temperature >180° C., morepreferably in a range from 200 to 300° C. and most preferably in a rangefrom 230 to 280° C.

In the process according to the present invention it is preferred thatas a process feature the space velocity of propylene (SVp) in the 2^(nd)catalytic oxidation reactor is at least 160 h⁻¹. The propylene oxygenmolar ratio in the 2^(nd) catalytic oxidation reactor is in the range of0.1-0.9 at a propylene conversion of at least 90 mol-% at one propylenepass through with a selectivity of acrolein and AA with respect topropylene of at least 90 mol-% in the 1^(st) catalytic oxidation stage.The acrolein conversion in the 2^(nd) catalytic oxidation stage is atleast 95 mol-% and the overall selectivity is at least 83 mol-%. It isfurther preferred that in the process according to the present inventionall the above process features are fulfilled.

The product gas resulting from the 2^(nd) catalytic oxidation reactionstage is subjected to the bottom of a quench tower. In this quenchtower, the AA is recovered from this product gas stream and withdrawnfrom the quench tower as an aqueous solution, which is then treated in asubsequent water/acrylic acid separation step. The process vent gastogether with non-converted propylene and/or propane leaves the quenchtower at the top and is then subjected to a catalytic combustion unit.

The AA produced together with process water and by-products is separatedfrom the process water in a subsequent water separation column (e.g. byazeotropic distillation). This separated process water is recycled tothe top of the quench tower where it is vaporized. After mixing with thevaporized process water, the vent gas is fed into the catalyticcombustion unit, where organic compounds in the process vent gas areoxidized to CO₂. The concentration of the total organic residue (carbondioxide, carbon monoxide, propylene, propane, acetic acid, acrylic acid,acrolein) in the process vent gas can range from 2 to 4 Mol-% and theconcentration of acrylic acid in the total organic residue from 1 to 3Mol-%.

According to the present invention, it is preferred that the quenchtower is operated under steady state conditions, the aqueous solutionwithdrawn from the quench tower comprises >55% by weight,preferably >90% by weight AA.

In order to operate the quench tower with a rather small number of traysand to remove AA, acrolein, other impurities and water from the quenchtower via the process vent gas, the temperature and the pressure at thetop of the quench tower should be adjusted to 30-90° C., preferably40-80° C. and 1-8 bar, preferably 1.05-6 bar, more preferably 1.1-1.5bar, respectively.

The quench tower comprises a cooling and an absorption section, whereinthe product gas is subjected to the cooling section in a product gasportion and a side stream leaves the quench tower in a portion abovesaid product gas portion. The quench tower comprises two parts, an upperand a lower part. The upper part of the quench tower comprisespreferably 20-40 theoretical plates. In a preferred embodiment of thepresent invention, these plates are realized with a packing like SulzerBX, Montz-PAK type BSH or with Koch-Glitsch Gauze Packing BX.Montz-Thormann-plates can also be used. It is preferred that the lowerpart comprises an indirect cooling section and that the upper partcomprises a direct cooling section. In the lower part of the quenchtower the temperature of the gas from the 2^(nd) catalytic oxidationreaction stage is reduced, preferably to 70-90° C., by recycling a partof the sump in the lower part of the quench tower. Preferably this socalled recycling stream has 10-60 times the amount of the aqueoussolution stream being withdrawn from the sump of the quench tower forpurification of the AA. The recycling stream is cooled in a heatexchanger to 60-80° C. The lower part of the quench tower is preferablyequipped with spray nozzles and segment cascade trays, random packing orstructured packing. However, the numbers of sections of the quench toweris not limited to two.

The side stream of water with AA is preferably taken out in the upperhalf, preferably upper third of the quench tower more preferably betweentray No. 4 and No. 8 and most preferably No. 5 and No. 6 forcircumvention of the pinch effect. This side stream is separated from atleast a part of AA in a subsequent separation unit (preferable in anazeotropic distillation column) and then sent back to the top of thequench tower together with the process water main stream.

The process vent gas obtained from the quench tower is subjected to acatalytic combustion unit. The catalyst used in the catalytic combustionunit is preferably based on TiO₂ carriers. Even more preferably are pureanatase carriers. These carriers are very resistant against abruptchanges in the process conditions like temperature, pressure etc. Thecatalytic combustion reaction involves the handling of large volumes ofoff gas under recycling, so very low pressure losses within thecatalytic combustion reactor is required (<10 mbar/m). This is achievedby the application of a honeycomb monolith carrier. This carrier isequipped with precious metal in a thin shell evenly distributed on thesurface of the channels.

The impregnation of the carrier is carried out with a moderatelyunstable precious metal compound, establishing an even distribution ofthe precious metal within the channels of the carrier with a lowpenetration depth in the carrier. This is carried out e.g. byimpregnation with a platinum nitrate solution, received by dissolutionof hexachloroplatinic acid in concentrated nitric acid. A co-precipationof a second noble metal e.g. Pd is carried out correspondingly.

The catalytic combustion reaction of the said process vent gas takesplace preferably at 175 to 650° C., more preferably at 190 to 580° C.and most preferably at 230 to 550° C. The combustion rate of the saidPt-catalyst is preferably in the range from 100 to 500 and morepreferably in the range from 100 to 250 g organic carbon/g Pt*h.

After the vent gas has been subjected to the catalytic combustion unitit is at least partially recycled to the gas mixture being oxidized inthe 1^(st) catalytic oxidation reaction stage with a recycle ratiobetween 0.1 to 0.4 (recycle ratio=molar ratio of recycle gas stream/feedgas stream). In a preferred embodiment of the present invention, theamount of combusted, recycled vent gas is adjusted in such a way that noadditional steam has to be added to the 1^(st) and 2^(nd) catalyticoxidation reaction stage.

The process according to the present invention has the advantage thatthe aqueous solution removed from the AA recovery step has a very highAA concentration so that the subsequent AA separation can be operatedwith a low energy consumption rate. Additionally, since all water fromthe AA separation is recycled to the AA recover step, no waste water isproduced. In a preferred embodiment of the present invention noadditional steam has to be added to the process, neither to the 1^(st)catalytic oxidation reaction stage nor to the 2^(nd) catalytic oxidationreaction stage.

A preferred embodiment of the process according to the present inventionis shown in FIG. 1.

In the embodiment of the present invention shown in FIG. 1, propylene 1,air 2, compressed by air compressor 101 and an inert combusted processvent gas (recycle) 3, comprising mainly N₂, CO₂ and steam, are combinedto a gas mixture 4 as inlet stream for the first stage oxidation reactor102. If the steam concentration in the inert combusted process vent gas3 is not high enough (e.g. during start up), additional steam 5 can beadded from an outside steam source to the first stage inlet stream 4.The steam and the inert gas concentration in the gas mixture 4 have tobe such that the gas mixture 4 is outside flammable limits.

The gas mixture 4 is fed into the 1^(st) catalytic oxidation reactor102, where propylene is converted mainly to acrolein. The effluent gasfrom the 1^(st) catalytic oxidation reactor 102 is subjected to a 2^(nd)catalytic oxidation reactor 103 in which acrolein is converted mainlyinto AA. If needed, the oxygen concentration in the feed gas 6 can beincreased by addition of air 2. Additional steam 5 can be added too, ifnecessary (e.g. during start up). The product gas stream 7 resultingfrom the 2^(nd) catalytic oxidation reactor 103 is cooled down in a heatexchanger 104 and then subjected to a quench tower 105.

From the sump of the quench tower an aqueous AA solution 9 comprisingprocess water and the recovered AA is withdrawn and then subjected to awater separation (not shown). A part of the aqueous AA solution 8 issubjected to a cooling device 106 and fed to the lower section. In thewater separation, the aqueous solution stream 9 is separated into an AAstream and a stream comprising process water and impurities with a lowboiling point, which are sent back to the quench tower via line 10.

In the quench tower 105 the AA is absorbed by the recycled process waterform the water separation. It is a feature according to a preferredembodiment of the invention that all process water leaving the quenchtower via line 9 to the water separation unit is recycled to the quenchtower via line 10. Optionally, a polymerization inhibitor 11 can beadded to the process water.

From one of the trays in the upper section of the top of the quenchtower an AA containing side stream 13 is sent to the subsequent waterseparation unit—e.g. the azeotropic water separation unit alreadycited—the AA removed from the product stream and the separated watersent back to the upper section of the quench tower via line 10.

The process vent gas 14 from the quench tower is subjected to acatalytic combustion unit 107 and a part of the combusted process ventgas 3 is recycled after cooling to the 1^(st) catalytic oxidationreactor 102 as steam and inert gas supply. The rest of the combustedvent gas is discharged into the atmosphere via line 16. Additional air15 can be added to the process vent gas 14 to adjust the combustionconditions.

The following Examples describe the present invention into more detail:

EXAMPLE 1

To a two stage oxidation reactor as described in FIG. 1 a gas mixture(feed gas) containing 0.990 kg/h of propylene (SV_(p)=166 h⁻¹, relatedto second reaction stage) and 4.890 kg/h of humidified air including0.220 kg/h of water was introduced to the first stage reactor (propylene10.1 Mol-%, oxygen (as air) 15.5 Mol-%, water 6.0 Mol-%). Additionally1.650 kg/h of dry air was introduced to the second stage reactor. Theoxidation reactors were charged with a commercially available Mo—Co—Bibased catalyst in the first stage and with a Mo—V—W based catalyst inthe second stage.

Under the following conditions the yield of AA was 86.36 Mol-%, theconversion of propylene was 97.54 Mol-% and the conversion of acroleinwas 97.57 Mol-%:

High temperature salt bath temperature 1^(st) stage: 335.4° C. Hightemperature salt bath temperature 2^(nd) stage: 296.3° C.

The top of the quench tower was charged with 0.939 kg/h of process waterincluding 0.005 kg/h of hydroquinone, resulting in a top temperature of52.6° C. These conditions led to an AA concentration of 83.9% by weightin the bottom of the quench tower including 1.18% by weight of dimer AA.

The side stream taken out between tray 5 and 6 of the quench tower(1.164 kg/h) contains 5.89% by weight of AA and 0.83% by weight ofacetic acid, from which after separation 0.939 kg/h of process water aresent back to the top of the quench tower.

To achieve complete catalytic combustion of the process vent gas 0.585kg/h of dry air were added to the catalytic combustion unit. Thecombusted waste gas consists of 4.27 vol.-% of oxygen, 80.9 vol.-% ofnitrogen, 3.72 vol.-% of carbon dioxide and 11.1 vol.-% of water. Fromthis combusted waste gas stream 0.729 kg/h were fed as recycle gas intothe first reaction stage. The molar ratio of recycle gas stream to feedgas stream was 0.16 leading to the following molar ratios of the feedgas composition at the first and second reactor inlet:

Feed gas composition at the first and second reactor inlet:

oxygen (1^(st) stage)/propylene: 1.51 oxygen (2^(nd) stage)/propylene:0.51 water (1^(st) stage)/propylene: 0.52 recycle gas stream (1^(st)stage)/feed gas stream: 0.16

The space time yield obtained for AA was 0.230 kg/(1_(R)h)¹). The carbonbalance is closed with a value of 0.7 Mol-%.

¹⁾R=total reaction volume of first and second stage reactors

EXAMPLE 2

To a two stage oxidation reactor as described in FIG. 1 a gas mixture(feed gas) containing 0.947 kg/h of propylene (SV_(p)=160 h⁻¹, relatedto second reaction stage) and 4.733 kg/h of humidified air including0.687 kg/h of water was introduced to the first stage reactor (propylene10.0 Mol-%, oxygen (as air) 15.3 Mol-%, water 17.0 Mol-%). Additionally1.583 kg/h of dry air was introduced to the second stage reactor. Theoxidation reactors were charged with a commercially available Mo—Co—Bibased catalyst in the first stage and with a commercially availableMo—V—W based catalyst in the second stage.

Under the following conditions the yield of AA was 88.44 Mol-%, theconversion of propylene was 98.73 Mol-% and the conversion of acroleinwas 98.86 Mol-%:

High temperature salt bath temperature 1^(st) stage: 353.6° C. Hightemperature salt bath temperature 2^(nd) stage: 278.4° C.

The top of the quench tower was charged with 0.537 kg/h of process waterincluding 0.008 kg/h of hydroquinone, resulting in a top temperature of51.7° C. These conditions led to an AA concentration of 68.8% by weightin the bottom of the quench tower including 0.57% by weight of dimer AA.Condensed phase of the process vent gas leaving the top of the quenchtower (0.275 kg/h) contains 3.12% by weight of AA, the loss of AA yieldvia the top of the quench tower was 0.53 Mol-%.

The side stream taken out between tray 5 and 6 of the quench tower(0.879 kg/h) contains 14.4% by weight of AA and 0.71% by weight ofacetic acid, from which after separation 0.450 kg/h are sent back to thetop of the quench tower.

Feed gas composition at the first and second reactor inlet:

oxygen (1^(st) stage)/propylene: 1.53 oxygen (2^(nd) stage)/propylene:0.51 water (1^(st) stage)/propylene: 1.70 recycle gas stream (1^(st)stage)/feed gas stream: 0

The space time yield obtained for AA was 0.226 kg/(1_(R)h)¹). The carbonbalance is closed with a value of 2.86 Mol-%.

¹⁾R=total reaction volume of first and second stage reactors

EXAMPLE 3

To a two stage oxidation reactor as described in FIG. 1, a gas mixture(feed gas) containing 0.965 kg/h of propylene (SV_(P)=163 h⁻¹, relatedto second reaction stage) and 4.923 kg/h of humidified air including0.173 kg/h of water was introduced to the first stage reactor.Additionally 1.304 kg/h of dry air was introduced to the second stagereactor. The oxidation reactors were charged with a commerciallyavailable Mo—Co—Bi based catalyst in the first stage and with acommercially available Mo—V—W based catalyst in the second stage.

Under the following conditions the yield of AA was 85.20 Mol-%, theconversion of propylene was 97.40 Mol-% and the conversion of acroleinwas 98.95 Mol-%:

High temperature salt bath temperature 1^(st) stage: 337.5° C. Hightemperature salt bath temperature 2^(nd) stage: 289.0° C.

The top of the quench tower was charged with 1.116 kg/h of process waterincluding 0.045 kg/h of hydroquinone, resulting in a top temperature of54.6° C. These conditions led to an AA concentration of 89.0% by weightin the bottom of the quench tower including 1.1% by weight of dimer AA.The condensed phase of the process vent gas leaving the top of thequench tower contains 3.00% by weight of AA, the loss of AA via the topof the quench tower was 1.19 Mol-%.

The side stream taken out between tray 5 and 6 of the quench tower (1.21kg/h) contains 14.6% by weight of AA and 1.24% by weight of acetic acid.

To achieve complete catalytic combustion of the process vent gas 0.183kg/h of dry air were added to the catalytic combustion unit. Thecombusted waste gas consists of 2.51 vol.-% of oxygen, 82.8 vol.-% ofnitrogen, 4.40 vol.-% of carbon dioxide and 10.3 vol.-% of water. Fromthis combusted waste gas stream 0.730 kg/h were fed as recycle gas intothe first reaction stage. The molar ratio of recycle gas stream to feedgas stream was 0.13 leading to the following molar ratios of the feedgas composition at the first and second reactor inlet:

Feed gas composition at the first and second reactor inlet:

oxygen (1^(st) stage)/propylene: 1.56 oxygen (2^(nd) stage)/propylene:0.41 water (1^(st) stage)/propylene: 0.42 recycle gas stream (1^(st)stage)/feed gas stream: 1.49

The space time yield obtained for AA was 0.221 kg/(l_(R)h)¹). Whencomparing with the example 2, the inlet water concentration has beenreduced from a molar ratio of 1.7 (water (1^(st) stage)/propylene) to0.42.

¹⁾R=total reaction volume of first and second stage reactors

1. A process for the production of acrylic acid (AA) comprising thesteps: (a) subjecting a first gas mixture comprising propylene, oxygen,an inert gas, and steam to a first catalytic oxidation reaction stagethereby converting the propylene in the presence of a catalyst mainlyinto acrolein being contained in a second gas mixture from said firstcatalytic oxidation reaction, (b) subjecting said a second gas mixturefrom the first catalytic oxidation reaction stage to a second catalyticoxidation reaction stage thereby converting the acrolein in the presenceof a catalyst mainly into AA, being contained in a product gas, (c)subjecting said product gas to a quench tower, wherein said AA isrecovered as an aqueous solution comprising AA being contained in theprocess water, wherein a process vent gas is obtained at the top of saidquench tower, and wherein the process vent gas is treated in asubsequent thermal or catalytic combustion unit, and (d) separating saidprocess water in a subsequent separation unit and process water is fedback into said quench tower so that most parts of the process watervaporized is mixed with the process vent gas leaving the quench tower ontop and is treated together with the process vent gas in the thermal orcatalytic combustion unit, wherein said first gas mixture has asteam/propylene ratio of greater than about 0.3 and less than about 2;and the amount of said process water is less or equal to the amount ofwater in said aqueous solution withdrawn from the quench tower; andwherein said treatment in said thermal or catalytic combustion unityields a combusted process vent gas and at least a part of saidcombusted process vent gas is recycled to said first catalytic oxidationreaction stage.
 2. The process of claim 1 wherein said catalyst of saidfirst catalytic oxidation stage is a Mo—Co—Bi based catalyst.
 3. Theprocess of claim 1 wherein said catalyst of said second catalyticoxidation stage is a Mo—V—W based catalyst.
 4. The process of claim 1wherein a carrier for the catalysts in the catalytic combustion unitinclude TiO₂ carriers.
 5. The process of claim 4 wherein said carrier isin the shape of honeycombs with a low delta pressure of less than about20 mbar/m³.
 6. The process of claim 1 wherein the oxygen concentrationin the second gas mixture can be increased by the addition of air. 7.The process of claim 6 wherein additional steam is added to the secondgas mixture.
 8. The process of claim 1 wherein the combusted vent gas isat least partially recycled to the gas mixture being oxidized in thefirst catalytic oxidation reaction stage.
 9. The process of claim 1wherein the concentration of the propylene in the gas mixture at thefirst catalytic oxidation reaction stage inlet is at least about 9Vol.-%.
 10. The process of claim 1 wherein said process water is atleast a part, of the water from a separation of said AA comprisingaqueous solution into water and AA.
 11. The process of claim 1 whereinsaid quench tower comprises a cooling and an absorption section, whereinsaid product gas is subjected to said cooling section in a product gasportion and a side stream leaves the quench tower in a portion abovesaid product gas portion.
 12. The process of claim 1 wherein a sidestream of water with AA is taken out in the upper half, wherein saidside stream is separated from at least a part of AA in a subsequentseparation unit then sent back to the top of the quench tower togetherwith the process water main stream.
 13. The process of claim 1 whereinthe space velocity of propylene (SVp) in said second catalytic oxidationreactor is at least 160 h⁻¹ or the propylene oxygen ratio is in saidsecond catalytic oxidation reactor is in the range of from about 0.1 toabout 0.9 at a propylene conversion of at least about 90 Mol-% at onepropylene pass through with a selectivity of acrolein and AA withrespect to propylene of at least about 90 Mol-% in said first catalyticoxidation stage and the acrolein conversion in said second catalyticoxidation stage is at least about 95 Mol-% and an overall selectivity ofat least about 83 Mol-%.
 14. The process of claim 1 wherein thetemperature at the top of the quench tower is in the range of from about30 to about 90° C.
 15. The process of claim 1 wherein the pressure atthe top of the quench tower is from about 1 to about 8 bar.
 16. Theprocess of claim 4 wherein said carrier is in the shape of honeycombswith a low delta pressure of less than about 10 mbar/m³.
 17. The processof claim 4 wherein said carrier is in the shape of honeycombs with a lowdelta pressure of less than about 3 mbar/m³.
 18. The process of claim 1wherein the concentration of the propylene in the gas mixture at thefirst catalytic oxidation reaction stage inlet is at least about 11Vol.-%.
 19. The process of claim 1 wherein the concentration of thepropylene in the gas mixture at the first catalytic oxidation reactionstage inlet is at least about 14 Vol.-%.
 20. The process of claim 1wherein the temperature at the top of the quench tower is in the rangeof from about 40 to about 80° C.
 21. The process of claim 1 wherein thepressure at the top of the quench tower is from about 1.05 to about 6bar.
 22. The process of claim 1 wherein the pressure at the top of thequench tower is from about 1.1 to about 1.5 bar.